The present invention relates to a piston crank mechanism of a reciprocating internal combustion engine, and particularly to a variable compression ratio mechanism of a reciprocating piston engine capable of varying the top dead center (TDC) position of a piston by means of a multiple-link type piston crank mechanism.
In order to vary a compression ratio between the volume in the engine cylinder with the piston at bottom dead center (BDC) and the volume with the piston at top dead center (TDC) depending upon engine operating conditions such as engine speed and load, in recent years, there have been proposed and developed multiple-link type reciprocating piston engines. In multiple-link type piston crank mechanisms (multiple-link type variable compression ratio mechanisms), a linkage is generally composed of three links, namely an upper link, a lower link, and a control link. One end of the upper link is connected via a piston pin to a reciprocating piston. The lower link is connected to the other end of the upper link. The lower link is also linked to a crankpin of an engine crankshaft. The control link mechanically links the lower link to a body (e.g., a cylinder block) of the engine, so as to restrict the degree of freedom of the lower link. The center of oscillating motion of control link 7 is designed to be controlled depending upon engine operating conditions. By changing or shifting the center of oscillating motion of the control link, the attitude of the lower link changes, thus varying the TDC position of the piston and consequently varying the compression ratio of the engine. Such multiple-link type variable compression ratio mechanisms have been disclosed in Japanese Patent Provisional Publication No. 9-228858 (hereinafter is referred to as xe2x80x9cJP9-228858xe2x80x9d) and in page 19 of the December issue for 1976 of the design engineering news xe2x80x9cProduct Engineeringxe2x80x9d. In the multiple-link type variable compression ratio mechanism, the lower link oscillates around the connecting point (serving as a fulcrum point or a pivot point) between the lower link and the control link, as the crankpin swings or rotates in a circle during rotation of the crankshaft. Thus, the lower link (intermediate link) functions to transmit the motion of the crankpin to the piston, while multiplying the displacement of the crankpin. The piston stroke is 2 times or more the radius of a crank, on the principle of lever-and-fulcrum or leverage. In other words, the ratio of piston stroke to the diameter of revolution of the crankpin (or the ratio of piston stroke to crank radius) can be multiplied via the lower link. Therefore, a great combustion load (combustion pressure) or inertial force acts upon the lower link in its bending direction through the piston pin and the upper link.
In a variable displacement engine disclosed in JP9-228858, an oscillating or rockable lever (called a bridge and corresponding to a lower link) is provided between a control arm (called a rocking arm and corresponding to a control link) and a connecting rod (corresponding to an upper link), for the purpose of varying the TDC position of a piston by an oscillating motion of the so-called bridge, thereby varying the compression ratio. However, in the variable displacement engine disclosed in JP9-228858, the connecting point between the upper and lower links, the connecting point between the control link and the lower link, and the crankpin axis are substantially aligned with each other. This lower link is formed as an elongated intermediate link. The structural design of the lower link (bridge) is insufficient from the viewpoint of a flexural rigidity. To enhance the flexural rigidity of the lower link, as disclosed in page 19 of the December issue for 1976 of the design engineering news xe2x80x9cProduct Engineeringxe2x80x9d, it is desirable to design the shape and dimensions of the lower link such that the connecting point between the upper and lower links, the connecting point between the control link and the lower link, and the crankpin axis are laid out at the respective vertexes of a triangle. However, in the multiple-link type variable compression ratio mechanism (multiple-link type piston crank mechanism) using the lower link having such a triangular layout of the three connecting portions, there is an increased tendency for second-order frequency components of the lower link to be created undesirably. Assuming that the multiple-link type piston crank mechanism with the lower link having the previously-noted triangular layout is mounted on an automotive vehicle, undesirable noise (containing booming noise) and vibration may be perceived audibly in the vehicle compartment. This deteriorates noise/vibration attenuation characteristics during operation of the engine. The inventors of the present invention discover that it is possible to effectively attenuate second-order vibration frequency components created owing to the lower link having the triangular layout by properly setting a position of the center of gravity of the lower link.
It is, therefore in view of the above-mentioned disadvantages, an object of the present invention to provide a multiple-link type piston crank mechanism of a reciprocating internal combustion engine in which second-order vibration frequency components created due to a lower link constructing part of the multiple-link type piston crank mechanism and having a triangular layout of three connecting portions can be effectively attenuated by way of proper setting of a position of the center of gravity of the lower link.
In order to accomplish the aforementioned and other objects of the present invention, a piston crank mechanism of a reciprocating internal combustion engine including a piston moveable through a stroke in the engine and having a piston pin and a crankshaft changing reciprocating motion of the piston into rotating motion and having a crankpin, the piston crank mechanism comprises an upper link connected at one end to the piston via the piston pin, a lower link connected to the other end of the upper link via a first connecting pin and connected to the crankpin, a control link connected at one end to the lower link via a second connecting pin and pivoted at its other end to a body of the engine to permit oscillating motion of the control link on the body of the engine, a center-of-gravity of the lower link lying inside of a triangle defined by three vertexes corresponding to a center of the first connecting pin, a center of the second connecting pin, and a center of the crankpin, and a distance from the center-of-gravity to the center of the crankpin is less than at least one of a distance from the center-of-gravity to the center of the first connecting pin and a distance from the center-of-gravity to the center of the second connecting pin.
According to another aspect of the invention, a piston crank mechanism of a reciprocating internal combustion engine including a piston moveable through a stroke in the engine and having a piston pin and a crankshaft changing reciprocating motion of the piston into rotating motion and having a crankpin, the piston crank mechanism comprises an upper link connected at one end to the piston via the piston pin, a lower link connected to the other end of the upper link via a first connecting pin and connected to the crankpin, a control link connected at one end to the lower link via a second connecting pin and pivoted at its other end to a body of the engine to permit oscillating motion of the control link on the body of the engine, a triangle being defined by three line segments, namely a first line segment between and including a center of the crankpin and a center of the first connecting pin, a second line segment between and including the center of the crankpin and a center of the second connecting pin, and a third line segment between and including the center of the first connecting pin and the center of the second connecting pin, a first center-of-gravity of the lower link, which is determined by a self-weight of the lower link itself except at least a weight of the first connecting pin and a weight of the second connecting pin, is laid out to be opposite to each of the first and second connecting pins with respect to the center of the crankpin, and a second center-of-gravity of the lower link, which is determined by an equivalent inertia weight obtained by adding the weight of the first connecting pin, the weight of the second connecting pin, a weight of a boss-shaped end of the upper link connected to the first connecting pin, and a weight of a boss-shaped end of the control link connected to the second connecting pin to the self-weight of the lower link, is laid out closer to the center of the crankpin than the first center-of-gravity.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.